The invention relates generally to circuitry for generating signal levels in response to the output of a photosensitive element and more particularly to circuitry for generating an output signal level in response to a photosensitive element operating in a high level ambient-energy environment.
It is well known in the literature and among circuit design engineers to design sensitive and satisfactory circuitry using for example photodiodes wherein accuracies of the order of one part in 10.sup.6 are not uncommon. In each of these systems, however, the measurements are made against relatively low DC background levels so that for example a signal on the order of 10.sup.-6 is made against a zero background level.
In apparatus such as that described in copending application Ser. Nos. 312,936 and 178,190, now U.S. Pat. No. 4,371,785, each assigned to the assignee of this invention, the disclosure of those applications being incorporated herein by reference, AC signal level whose RMS values are on the order of 100 or more decibels below the DC background level are detected. This corresponds to absorption level reference measured with the apparatus of 10.sup.-6 A.U. In this circumstance, presently available technology does not provide measurements with sufficient accuracy and stability to fully make use of the capabilities of an inexpensive photodiode detection system. In particular, prior art designs would lead one to employ circuit parameters which promote deterioration of the available signal precision by at least one or two orders of magnitude.
It should be noted however that a photomultiplier tube, which provides an internal amplification on the order of 10.sup.5, at low noise levels, provides a satisfactory noise signal although the DC bias saturates at this amplification level. It is only for photosensitive elements such as semiconductor or vacuum photodiodes (wherein external circuit elements are employed to provide the necessary amplification) that the substantial deterioration of the signal to noise figure occurs.
It is therefore an object of the present invention to provide an amplifier, and a method for selecting circuit components for the amplifier, for maximizing the signal to noise figure of the amplified output signal from a photosensitive element operating in a high ambient noise level environment. Other objects of the invention are a method for optimizing the output of a circuit element in order to provide maximum signal resolution in the presence of high ambient noise levels.
Further objects of the invention are an amplifier and a method for selecting amplifier circuit parameters, which enable high reliability, high precision, and low noise level to be obtained in connection with amplifying small AC signal levels from a photosensitive element, and in particular photodiodes, operating at high ambient DC input levels.